Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual é a aeronave para 21 tripulações?
 ###Tabelas: CREATE TABLE current_fleet (current_fleet_id AUTO_INCREMENT PRIMARY KEY, Length (feet) VARCHAR, Year VARCHAR, Make and model VARCHAR, Floor type VARCHAR, Number of seats INT, Bicycle capacity† INT, Fuel propulsion VARCHAR, Quantity INT) CREATE TABLE men's_doubles (men's_doubles_id AUTO_INCREMENT PRIMARY KEY, Location VARCHAR, Year INT, Champion VARCHAR, Runner-up VARCHAR, Score VARCHAR) CREATE TABLE import (import_id AUTO_INCREMENT PRIMARY KEY, # INT, Importing Nation VARCHAR, 10 3 bbl/day (2011) INT, 10 3 m 3 /day (2011) INT, 10 3 bbl/day (2009) INT, 10 3 m 3 /day (2009) INT, 10 3 bbl/day (2006) INT, 10 3 m 3 /day (2006) INT) CREATE TABLE tactical_organization (tactical_organization_id AUTO_INCREMENT PRIMARY KEY, Type of unit VARCHAR, Type of aircraft VARCHAR, Number of aircraft VARCHAR, Number of crews VARCHAR, Men per crew VARCHAR, Total personnel INT, Officers INT, Enlisted INT) CREATE TABLE u.s._nielsen_ratings (u.s._nielsen_ratings_id AUTO_INCREMENT PRIMARY KEY, Episode VARCHAR, Air Date VARCHAR, Rating INT, Share INT, Rating/Share 18–49 VARCHAR, Viewers (m) INT, Timeslot Rank INT, Night Rank VARCHAR, Overall Rank INT)  
SELECT Type of aircraft FROM tactical_organization WHERE Number of crews = 21